Spring clutch assemblies, such as that disclosed in U.S. Pat. No. 4,570,318 to Kish, are known for transferring torque from a drive member to a driven member, with spring clutch assemblies preferred as torque transmitting devices where overrunning conditions would be encountered. Such conditions occur when the driven member attains a higher rotational speed than the drive member and therefore declutching is required to prevent rotation of the driver. Overrunning spring clutches utilize a coil spring which expands radially under a driven load to couple two clutch members, an input member and an output member. Each clutch member includes a bore where the coil spring is located, with approximately one half of the spring in the input member and one half in the output member. The clutch spring is usually fixed at the drive end by press fitting onto an arbor which is attached to the input member. The arbor is coaxial with the input and output members, passing through the hollow core of the spring. The arbor serves as a centering means for alignment and support of the coil spring.
In the non-rotating state, a space is provided between the spring and the inner bore surfaces of the clutch members, except at the ends where "teaser" coils are in contact with the inner surfaces. These teaser coils ride on the surfaces to actuate the spring. When the input member begins to rotate, in a direction counter to the pitch of the spring coils, the teaser coils, dragging against the output member, cause the spring to unwind, expanding radially so that it is in driving contact with the bore surfaces along its entire length, transferring torque from the input to the output member.
In the overrunning condition, the driven member rotates faster than the drive member, in the direction of the spring pitch, driving the teaser coils to rewind the spring and disengage the driven member from the drive member. This may occur, for example, in a helicopter during engine shutdown when momentum maintains main rotor rotation.
A problem with spring clutches is their inability to be disengaged in a non-overrunning condition. For example, on a helicopter, it may be desirable to use the driver to power auxiliary devices or to conduct equipment checks, without rotating the rotors. Spring clutch assemblies operate automatically in response to rotation of the driver and are not disengageable on command. Additional devices, such as an auxillary clutch, may be used to disengage the driver, however, these would add excessive weight and complexity to the aircraft.